1. Field of the Invention
This invention relates generally to borehole logging apparatus and methods for performing nuclear radiation based measurements. More particularly, this invention relates to a new and improved apparatus for determination of formation porosity and gas saturation using a measurement-while-drilling (MWD) tool that measures formation density and formation porosity shortly after a well has been drilled.
2. Background of the Art
Oil well logging has been known for many years and provides an oil and gas well driller with information about the particular earth formation being drilled. In conventional oil well logging, after a well has been drilled, a probe known as a sonde is lowered into the borehole and used to determine some characteristic of the formations which the well has traversed. The probe is typically a hermetically sealed steel cylinder which hangs at the end of a long cable which gives mechanical support to the sonde and provides power to the instrumentation inside the sonde. The cable also provides communication channels for sending information up to the surface. It thus becomes possible to measure some parameter of the earth's formations as a function of depth, that is, while the sonde is being pulled uphole. Such “wireline” measurements are normally done in real time (however, these measurements are taken long after the actual drilling has taken place).
Porosity measurements are commonly done by using a dual detector neutron logging tool using a source of neutrons irradiating the formation being studied. In wireline applications, the tool typically is forced against one side of the borehole wall by eccentralizing springs. Other devices, such as laterally extending arms may be used. The eccentralizing device may be hydraulically, mechanically, electrically or electromechanically powered. The resulting neutron population is sampled by a pair of neutron detectors spaced at different distances from the neutron source. Using a dual detector tool, the ratios of measurements made by the near detector and the far detector are used to get a compensated porosity measurement. Alternatively, the so-called spine and rib method may also be used. The method works reasonably well in wireline applications.
Density measurements are commonly done by using a dual detector gamma ray logging tool using a source of gamma rays irradiating the formation being studied. Just as for the porosity tool, in wireline applications, the tool typically is forced against one side of the borehole wall by laterally extending arms. The resulting gamma ray population is sampled by a pair of gamma ray detectors spaced at different distances from the gamma ray source. Using a dual detector tool, measurements made by the near detector and the far detector for a density logging tool are also corrected using the so-called spine and rib method to get a corrected porosity measurement.
Neutron and density logging tools have different responses to the presence of gas in the formation because of differences in the physics of the measurements. A neutron tool response is sensitive mainly to the number of hydrogen atoms in the formation. During calibration of a neutron porosity tool, water-filled formations are used to develop porosity algorithms. A decrease in the number of hydrogen atoms is equivalent to a lower porosity. If a neutron porosity tool is then used in a gas-filled formation (which has a lower number of hydrogen atoms than a water-filled formation of the same porosity), the porosity estimate will be lower than the true porosity.
Gamma ray tools, on the other hand, are responsive to the total number of electrons in the formation. When a gamma ray tool is used in a gas formation, the estimated porosity will be higher than the true porosity. Hence in the case of a reservoir where there is gas instead of water or oil in the pore space, the porosity estimates made from the neutron logging tool and the gamma ray logging tool are different. Under these conditions, the true formation porosity lies between the measured neutron and density values. Log interpreters often find it difficult to accurately estimate the true formation porosity from these two curves.
The process is further complicated by the effects of borehole fluid invasion. The effect of invasion is to force the gas from the formation and replace it with borehole fluid. The neutron tool begins to sense the presence of more hydrogen atoms and yields a porosity estimate that is higher than before when only the gas was present; the opposite occurs for the density tool. The increase in the amount of water in the near formation, i.e., the increase in the number of electrons, is interpreted by the density tool algorithm as a higher density which translates into a lower porosity estimate. The end result is that the separation between the two curves begins to disappear as the invasion front increases in radial depth. The rate at which the two porosity logs approach the true porosity depends upon their radial sensitivities and their respective depths of investigations (DOI). In wireline measurements (made at some time after drilling has been completed), a reasonably correct estimate of porosity may be obtained by either tool. However, in measurement-while-drilling logging tools, invasion effects are minimal and the resulting porosity estimates would be in error.
U.S. Pat. No. 4,810,459 to Fontenot teaches s method and apparatus for determining true formation porosity utilize downhole measurement-while-drilling neutron porosity measurement devices. An initial measurement is taken shortly after the formation is bored and before any substantial invasion by the drilling fluid occurs. Subsequent measurements are made until a steady, no longer increasing, measurement is reached indicating saturation of the formation by the drilling fluid to the depth of the measurements. The steady measurement is indicative of the true porosity of a gas containing formation while the difference between the initial and steady measurements is indicative of the gas saturation of a gas containing formation. A major problem with the method of Fontenot is that measurements must continue to be made until this equilibrium is established. This is contrary to one of the reasons MWD measurements are made, which is to get an estimate of formation characteristics as soon as possible. Waiting for equilibrium to be reached is expensive in terms of rig costs. The Fontenot method does not address problems caused by tool rotation and variable standoff on a rotating sensor assembly.
U.S. Pat. No. 5,684,299 to DasGupta teaches a method of determination of formation porosity of a partially invaded gas reservoir by averaging density and neutron tool porosities. During the method of this invention, density tool and neutron tool porosity measurements are taken of the formation. These porosity measurements are fitted to a predetermined porosity to determine the percentage of each porosity measurement that will be summed to estimate the formation porosity of the gas reservoir. The percentages of the density and neutron porosity measurements are dependent on a correction factor that is determined from the fitting process. The method of DasGupta requires a reference calibration porosity and furthermore does not provide an estimate of gas saturation of the reservoir. Commonly used acoustic methods for determination of gas saturation are generally of marginal value since even a small amount of gas (low saturation gas or LSG) in the formation significantly affects the velocity of propagation of compressional waves.
There is a need for an accurate method of determination of porosity and gas saturation in an earth formation using a MWD apparatus that is insensitive to invasion effects. The present invention satisfies this need.